1. Field of the Invention
The present invention relates to methods for enhancing the mechanical properties of polymers such as ultra-high molecular weight polyethylene (UHMWPE). More particularly, the present invention relates to a method for crosslinking UHMWPE to increase its wear resistance in orthopaedic bearing applications.
2. Description of the Related Art
UHMWPE is commonly used as a bearing material paired with an opposing metal component in orthopaedic implants such as hips and knees. It is the high molecular weight of the polymer that imparts the desirable characteristics for implants, such as high impact strength and abrasion resistance. It is known that irradiating certain polymers such as UHMWPE can cause changes in its chemical and mechanical properties. For example when UHMWPE is subjected to gamma irradiation in the range of 3.0-5.0 Mrads, it has been observed that with time it can change color and become embrittled. This is of interest in the medical device field since such an irradiation dose is within the range of commonly used sterilization processes. The general belief is that the changes in material properties are due to competing reaction pathways, one being crosslinking within and between polymer chains, and another being oxidation. Crosslinking results in an increase in molecular weight of the polymer, while oxidation results in decreasing molecular weight. High energy, ionizing radiation, such as gamma or electron beam radiation, breaks molecular bonds, called chain scission, and creates free radicals that are highly reactive species. The severed chains can recombine, crosslink with adjacent chains, or combine with other species such as oxygen. In the presence of oxygen, the severed chain is more likely to form an oxygenated species which is then not able to form crosslinks or recombine, resulting in a reduction of molecular weight. It is the reduction of molecular weight that causes a reduction in mechanical properties and embrittlement. Some of the free radicals formed are not capable of reacting due to location in the polymer structure and thus can exist in the polymer for long periods. The migration of such species as oxygen over long periods of time to these isolated free radicals can result in further oxidation and molecular weight reduction, with a subsequent time dependent degradation of properties.
Crosslinking is known to increase the abrasion resistance of polymers. In orthopaedics it has been indicated as one way to increase the wear life of UHMWPE implants. Crosslinking occurs in polymers when adjacent polymer chains form cxe2x80x94c bonds. Such crosslinking acts to prevent the polymer chains from being pulled or pushed apart. The degree of crosslinking of a material is a function of the radiation dose it receives. The total dose received depends on the penetrative properties of the radiation in the material being treated and the exposure time to the radiation source. State of the art electron beam radiation sources are capable of relatively high dose rates. Thus, relatively short exposure times can yield relatively high doses. Because of this, electron beam radiation facilities typically position a radiation source directed at a moving conveyor on which samples are moved under the electron beam source at a speed selected to give the desired dose. The high dose rates of electron beam irradiation is believed to result in less oxidation than slower irradiation sources such as gamma irradiation, due to the increased availability of oxygen over the longer exposure time of low dose rate irradiation sources.
Some investigators have proposed ways to reduce oxidation and/or increase crosslinking. Their methods generally involve maintaining the article being irradiated in an oxygen free environment. For example, Shen and Dumbleton teach that gamma irradiation in an argon atmosphere results in a high percentage of crosslinking and improves the wear performance of polyethylene. C. Shen and J. H. Dumbleton, The Friction and Wear Behavior of Irradiated Very High Molecular Weigh Polyethylene, 30 Wear, 349 (1974). Grobbelaar et al. teach that by gamma irradiating polyethylene prostheses in a reactive organic atmosphere containing acetylene, enhanced crosslinking at the surface is achieved which results in reduced deformation while maintaining excellent abrasion resistance. Grobbelaar et al., The Radiation Improvement of Polyethylene Prostheses: A Preliminary Study, 60-B:3 JBJS 370 (1978).
Other investigators have taught free radical elimination via post irradiation processing. Kang et al. teach that crosslinking polyethylene with gamma irradiation is enhanced by raising the temperature of the polyethylene during irradiation and furthermore that free radicals can be removed by annealing the polyethylene after irradiation. Kang et al., The Radiation Chemistry of Polyethylene. IX Temperature Coefficient of Cross-Linking and Other Effects, 89:9 Journal of American Chemical Society 1980 (1967). Sun et al. likewise teach in U.S. Pat. No. 5,414,049 that free radicals may be removed by heating the irradiated article.
In addition to dealing with free radicals, problems may arise when irradiating thick parts with high dose rates. Early investigators used low dose rate gamma irradiation that easily penetrated through parts of various thicknesses and achieved desired dose levels over many hours of exposure to the gamma source. The present investigators have found it advantageous to use E-beam irradiation because of its very high dose rate and capability of reaching desired dose levels very quickly. However, orthopaedic applications may require parts up to 9 cm to be treated. A problem with irradiating parts over 4 cm with an electron beam is that an electrical charge potential can build up within the material being irradiated, and then discharge, leading to defects characterized by carbon tracking or scoring. This carbon tracking or scoring may be visible with the naked eye or only with the aid of magnification. The carbon scoring appears as black xe2x80x9clightning strikesxe2x80x9d in the polymer mass or as black marks on the polymer surface. Another problem that arises is that the energy level of the irradiation source determines the depth to which the irradiation will penetrate. With sources in the 10 MeV range, which is typical for commercial E-beam vendors, penetration occurs at approximately 4.0-5.5 cm. Furthermore, the dose delivered to the sample over this penetration depth is not uniform.
The present invention provides a method for irradiating polymers such as UHMWPE to produce property enhancing crosslinking. The method is particularly advantageous in that it can rapidly process parts of varying thickness while producing a desired dose distribution through the sample thickness. Likewise the method can process thick parts with minimal electrical discharge. Finally, the method includes steps to reduce or eliminate free radicals from the irradiated UHMWPE.
These advantages are achieved by a method which includes determining the desired dose distribution in the sample to be irradiated, determining the optimal thickness to produce the desired distribution with a particular electron source, determining whether a shield or shields will be necessary in combination with the sample to replicate the optimum thickness, and assembling the sample and shields and irradiating the assembly on one or more sides. In addition a variety of electrical discharge control methods and devices are disclosed.